Brief History and Background

The first known neurophysiologic recordings of animals were performed by Richard Caton in 1875. The advent of recording the electrical activity of human beings took another half century to occur. Hans Berger, a German psychiatrist, pioneered the EEG in humans in 1924. The EEG is an electrophysiological technique for the recording of electrical activity arising from the human brain. Given its exquisite temporal sensitivity, the main utility of EEG is in the evaluation of dynamic cerebral functioning. EEG is particularly useful for evaluating patients with suspected seizures, epilepsy, and unusual spells. With certain exceptions, practically all patients with epilepsy will demonstrate characteristic EEG alterations during an epileptic seizure (ictal, or during-seizure, recordings). Most epilepsy patients also show characteristic interictal (or between-seizure) epileptiform discharges (IEDs) termed spike (<70 μsec duration), spike and wave, or sharp-wave (70–200 μsec duration) discharges.

EEG has also been adopted for several other clinical indications. For example, EEG may be used to monitor the depth of anesthesia during surgical procedures; given its great sensitivity in showing sudden changes in neural functioning even as they first occur, it has proven quite helpful in this setting in monitoring for potential complications such as ischemia or infarction. EEG waveforms may also be averaged, giving rise to evoked potentials (EPs) and event-related potentials (ERPs), potentials that represent neural activity of interest that is temporally related to a specific stimulus. EPs and ERPs are used in clinical practice and research for analysis of visual, auditory, somatosensory, and higher cognitive functioning.

The EEG is thought to be primarily generated by cortical pyramidal neurons in the cerebral cortex that are oriented perpendicularly to the brain's surface. The neural activity detectable by the EEG is the summation of the excitatory and inhibitory postsynaptic potentials of relatively large groups of neurons firing synchronously. Conventional scalp or cortical surface–recorded EEG is unable to register the momentary local field potential changes arising from neuronal action potentials. Please see Appendix 1 for further details on neurophysiologic principles underlying the EEG.

An unfortunate reality of EEG is that cerebral activity may be overwhelmed by other electrical activity generated by the body or in the environment. To be seen on the scalp surface, the miniscule, cerebrally generated EEG voltages must first pass through multiple biological filters that both reduce signal amplitude and spread the EEG activity out more widely than its original source vector. Cerebral voltages must traverse the brain, CSF, meninges, the skull, and skin prior to reaching the recording site where they can be detected. Additionally, other biologically generated electrical activity (by scalp muscles, the eyes, the tongue, and even the distant heart) creates massive voltage potentials that frequently overwhelm and obscure the cerebral activity. Temporary detachments of the recording electrodes (called “electrode pop” artifact) can further erode the EEG, or even imitate brain rhythms and seizures. The bottom line is that biological and environmental electrical artifacts frequently interfere with the interpreter's ability to accurately identify both normal rhythms and pathological patterns. Fortunately, artifacts possess many distinguishing characteristics that are readily identifiable by well-trained, careful observers. Please see Appendix 4 for several examples of artifacts commonly encountered during EEG recording.

A typical EEG display graphs voltages on the vertical domain and time on the horizontal domain, providing a near real-time display of ongoing cerebral activity (Figure 1). With digital recording and review, the interpreter can change several aspects of the EEG display for convenience and intelligibility of the data. The interpreter is able to adjust the sensitivity (also known as [aka] “gain”) of the recording, in microvolts per millimeter, to either increase or reduce the display height of waveforms. One may also alter the amount of time displayed, which is sometimes referred to as an epoch and used to be known as “paper speed.” Shorter intervals can be viewed with a few seconds on a computer screen, a distinct advantage for viewing very brief EEG events such as epileptiform spikes. Conversely, the time scale may be expanded to display longer segments of EEG over several minutes to look at slowly evolving rhythmic discharges. Digital filters may also be applied to reduce artifact in certain settings but must be used with great caution since they also filter EEG activity of interest and may distort EEG waveforms severely.

Figure 1.

Normal EEG with typical montage. An example of the EEG recorded during wakefulness in a 24-year-old woman. This is a 10-second duration epoch. The first four channels, together referred to as a chain, show cerebral activity recorded from the midline head (more...)

EEG uses the principle of differential amplification, or recording voltage differences between different points using a pair of electrodes that compares one active exploring electrode site with another neighboring or distant reference electrode. Only through measuring differences in electrical potential are discernible EEG waveforms generated. By convention, when the active exploring electrode (termed G1, for “Grid 1,” a historical convention from analog amplification) is more negative than the reference electrode (G2), the EEG potential is directed above the horizontal meridian (i.e., an upward wave), whereas if the opposite is true, where the reference electrode is more negative, the EEG potential vector is directed below the horizontal meridian (downward potential). Other polarity possibilities are shown in Figure 2.

Figure 2.

Polarity conventions and localization in EEG. An upward deflection is surface negative, and a downward deflection is surface positive. Each derivation or channel is made up of two electrode site pairs, in the manner shown below, which shows a longitudinal (more...)

A related technique to the EEG is MEG, which does not record electrical activity but, rather, utilizes sensors to capture magnetic fields generated by the brain. MEG provides complementary information to the EEG by demonstrating the activity of magnetic cerebral dipoles. Since magnetic fields are less degraded by the head's biological filters than electrical activity, MEG dipoles may produce more accurate locations for cerebral epileptiform generators than EEG. A detailed review of MEG is beyond the scope of this review. The interested reader is referred to excellent recent literature on the subject (1–3). See Figure 3 for an example of MEG.

Figure 3.

Example of MEG. Equivalent current dipoles in a young girl with tuberous sclerosis. Color-coded regions of interest represent hand motor (red), somatosensory (blue), and epileptiform dipoles (aqua). The sagittal image demonstrates that epileptiform dipoles (more...)